first of all you are posting in HTML5 forum, so I am assuming that you want to make a HTML5 website.

HTML5 does not do frames.

I would suggest against making frameset webpages, not only is it an old and outdated way of doing things. you only have the html4.01 frameset doctype to work with, but they screw up majorly in search engines, as if someone searches for and finds your site, the chances of them getting the page WITHOUT the associated navigation frame/ header frame is extremely high.

My problem is : I want my web site to have a fixed width (1280 pixels) and I don't know how to do this with the frames even if I spent many hours...

If I run the site on a bigger resolution it will be aligned on the left side of the screen and on the right side will be a lot of empty space. I must to center it on the screen ...

i'd also suggest making the fixed width smaller (like 960px)

put the whole website in a wrapper with margin:0 auto; that'll centre it.